Selenium crystals and method of preparing the same



?atented Mayne, teas.

teaser meta care.

FAY C(LUFF) BROWN, 01E IOWA CITY, IOWA.

SELENIUM CRYSTALS AND METHOD OF PREPARING THE SAME.

Ho Drawing.

To all whom it may concern:

Be it known that I, FAY C. BnowN, a citizen of the United States,residing at Iowa City, in the county of Johnson and State of Iowa, haveinvented new and useful Selenium Crystals and Methods of Preparing theSame, of whichthe following is a speclfication.

This invention has reference to selenium crystals and its object is toproduce light sensitive selenium crystals of greatly increasedsensitiveness.

It has long been known that selenium in some of its forms is lessresistant to the passage of an electric current when under the influenceof light than when in the dark, that is, protected from the action oflight. Ad-

vantage has been taken of this property of selenium for various purposesand the devicesfor utilizing the selenium in such manner have beencustomarily known as selenium cells, but such term is somewhat of amisnomer, wherefore the term selenium bridge isused throughout thisspecification, becausethe selenium is utilized to bridge a gap in theelectric circuit, whereby the variable resistance'represented by theselenium ma be utilized, 7 1 l r ior to the present inventioncrystallized selenium has been produced, but the crystals have been ofcomparatively small size. For

this reason there can be no control of the arrangement ,of the crystalswhich because of their-extremely small size are'present in greatmultitudes in the ordinary selenium bridge. Such crystals have theiraxes pointing in all directions.

By the production of crystals of selenium of large size, that is,readily visible individually to the unaided eye and easy to handle,

it is quite possible. to produce a selenium bridge of suitable capacityfrom a single crystal, and the capacity may be increased by the emloyment of twoor three or more crystals, or the crystals are of size asto be readily oriented. It has been found that-a single hexagonalcrystal shows a greater sensibility to light when the illuminationimlinges on. the end of the crystal and-trave s in the direction of theprincipal axisof the crystal.

'Iaoreover, it has been found that such I crystals whenjsnbiectedtopressu-re'exhibit a many fold increase sensitiveness' to the action ot'lighnsolthat a selemum crystal so arrangedthat the .lightstrikes it inthe dismdi large Application filed March 7, 1916. Serial No. 82,782.

rection of its principal axis and under compression .to a considerabledegree, 18 increased in light sensitiveness, say, a thou-' sand timesover the sensitiveness of selenium bridges as heretofore constructed.

' The selenium bridge of the present invention, therefore may compriseone or more crystals of selenium with means for subjecting such crystalor crystals to pressure and at the same time having the crystals sopresented that the light rays strike them in the direction of theirprincipal axes.

' The invention will be best understood from a consideration of thefollowing detailed description.

In using selenium crystals, made after the manner herein described, forselenium bridges whose resistance varies in accordance with the amountof light impinging thereon, certain precautions should be observed, ifthe best results are to be secured. The selenium crystal is placed inthe circuit between the electrodes of any suitable material, forexample, those hereinafter mentloned. Increased sensitivity results fromapplying pressure to the crystal and in the case of hexagonal crystalsit is advantageous to apply the light ray in the direction of theprincipal axis of the crystal. The crystal may be shielded fromextraneous light in any suitable manner and a window or opening may beprovided through which the varying light ray may impinge upon thecrystal. For a more detailed description of the possible uses to whichsuch crystals may be put and of the precautions to be observed in usingthem reference may be made to the specification of my United StatesPatent 1,219,432, patented March Q0, 1917.

Selenium exists in the amorphous or nonconducting form, in the redcrystalline form, also non-conducting, and in a crystalline metallicstate with the crystals of monoclinic or hexagonal shape very sensitiveto light and with properties peculiar to the various axes of thecrystals and also due to the manses, wherefore the sealed receptacle contains nothing but selenium which becomes vaporized by the heat employedand from which vapor the crystals are deposited. The crystals are of themonoclinic form when the temperature is about 171 C. Hexagonal crystalsof a maximum sensibility to light are obtained when the vapor issublimed at a temperature of about 183 A high sensitiveness hasbeenobtained when occluded gases have been driven 0H from the selenium byrepeated boiling and condensing in a high vacuum. For some purposes itis advantageous to have argon, neon, helium and other gases mixed withthe vapor of selenium either separately or in combination, and, again,it is advantageous to form the crystals in an atmosphere of seleniumvapor only and afterward allow them to absorb any one gas'or a mixtureof the gases at temperatures below the temperature of formation.

While the temperatures named have been found to produce most excellentresults, it is to be understood that the invention is not in any mannerlimited to any precise temperatures. 1 I

One method of producing the herein described sensitive selenium crystalsof high sensitivity will be described in. detail. V treous seleniumwhich i s,to be transformed into crystals is .placed inone end of aglass tube of about mm. inside diameter and of about'30 to 60 cm.length. This tube is placed in a cylindrical electrical oven of about 30cm. length closed at one end. A suitable current is used to heat theselenium to a suficiently high temperature to cause it to vaporize. Theglass tube may'behlghly evacuated, filled with some of the various gaseshereinbefore mentioned, or filled with ordinary air at atmospheric orlower pressures. In general the use of air is not recommended, but itselimination is not essential for all purposes Where theg'lass tube1projects from the oven will be a region of igh temperature gradient.The apparatus should be set up and maintained quietly for a long periodwhich may vary in accordance with circumstances from a minimum of oneday to a maximum of one week or more. Along the part of the tube wherethe temper- .ature grad ent is the greatest will be found numerouscrystals.

In general the largest crystals will be formed at the point of highesttemperature at which the selenium will sublime. In various parts of theregion in which crystallization takes place'there. will occur variousforms of selenium crystals. It is not possible to determine in advanceor predict with certainty the form which the crystals will take in anyparticular instance or the relative quantities or sizes of the varioustypes, or the size of individual crystals. It will happen, however, thatcertain crystals will be relatively large, that is asses smallercrystals and the selenium which is deposited in non-crystalline form maybe resublimed either with or without the addition of other selenium. Theresublimation may be performed under thesame or difierent conditions asthe first sublimation as may be desired.

For further and more detailed information reference may be made to anarticle by me on the Crystal forms of metallic selenium and some oftheir hysical roperties, plublished in Physical view, S., Vol.4,

0. 2,1lu ust 1914.

A sing e crystal of a substance is more likely to be pure and of thesame crystalline structure throughout than is the case with aheterogeneous mass of material, wherefore such crystal has the advantageof a uniform molecular structure throu hout, that is, a uniform andregular spacing of the molecules. Such uniformity of structurecontributes to the stability of the crystal and of the selenium bridgein which the crystal is included.

Large crystals are advantageous in that because of the parallel planefaces of the crystal it is possible to apply pressure on the electrodesafter the crystals are formed.

The pressure on a single crystal or an assembla e of single crystals ismore likely to pro'.uce a uniform stress in the crystal or crystals thanis the case where pressure is applied to a mass-aggregate ofsub-microsco ic crystals.

arge crystals permit ressure to be applied with so simple a evice as ascrew, and also permit the control of the axis along which the crystalmay be illuminated. Not only is the sensibility difi'erent, dependentupon the orientation of the axis to beilluminated, but this orientationdetermines which region of the spectrum will produce the maximum changeof conductivity. I I Experience has shown great stability of the newcrystals of selenium over selenium as hertofore used in seleniumbridges. A selenium crystal as produced and employed in accordance withthe present invention has been exposed to most intense light fortwenty-four hours a day for thirty-five days without showingdeterioration. Such a crystal mounted for a year has now the samesensibility as when first mounted.

The electrodes may be made of a material 1 iso intense selenium vapor.It is not imperative that the light afiecting the bridge shall reach thecrystal or crystals in the direction of the principal ax s thereof. lForinstance,

are described in my V patent referred toabove.

What is claimed is 1. 'lhe method of reparing selenium for use inselenium hri ges, Which consists. in freein selenium from occluded see,then vaporizing the selenium by heat in a high vacuum, and causing thedeposition ofcrystals from the selenium vapor so produced.

yecu n a 5. e method of ma nng selemerysp 2. The method of preparinseleni crystals for use in selenium bri ges, which consists in freeingthe selenium from cccluded gases, then heating the selenium in a highvacuum to produce selenium vapor, and causing the deposition of crystalsof selenium from such selenium vapor, und

finally causing the 'crymal'sso formed to ehsorh one. or more gases.

3. Themethod of making selenium crystels by sublimation comprisingvaporizing the seleni in u' vucuum.

4. The method of memn seleni crystals trom selenium h 'subhmutioncomprising eliminating occ uded gases from the selenium and vaporizingthe selenium in a sion of undesired .crystels comprisin? e tals fromselenium which comprises vaporizing selenium end precipitating the veporunder conditions which prevent the occlugas in the resulting crystals."

65. The ethod of making seleni crystals from selenium comprisingvaporizing the selenii end precipitating the vapor at a temperuture ofapproximately 180 C.

7. A seleni cr stel having occluded therein inert gas su stantiallyexclusively. 8. A selenium crystal heving occluded therein gusprincipally of the helium group.

9. The method of making selenium crystals from selenium by sublimation,comprising vuporizing the selenium in a closed vessel at suhstentiallyless than atmospheric pressure, and maintaining the vessel under,

uniform conditions of temperature and pressure for a long period of timeto cause the deition of the selenium vapor in the to of the desiredcrystals.

1d. The method of preparing selenium repeatedly vaporizin and'condensingnium in an evacuate vessel.

11. The method of producing sensitive seleni crystals, comprisingrepeated suhlimation of selenium in a vessel from which to f the largerpert of the atmosphere has been rev otters nnovv u.

Witnesses:-

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